Arecibo Pings a Comet

Arecibo Pings a Comet

In May, the Camelopardalids meteor shower didn’t perform as hoped, with rates much lower than predicted. However, that doesn’t meant the entire event was wasted: Astronomers got some interesting observations of the comet 209P/LINEAR, the parent of the shower:

Ghostly radar images of the nucleus of the comet 209P/LINEAR reveal contours on the solid surface.

Photo by Arecibo Observatory/NASA/Ellen Howell, Patrick Taylor

These are not images taken in the usual way; the gigantic Arecibo radio telescope was used like a radar gun to send radio pulses at the comet, which then bounced back and were used to generate a map. Emily Lakdawalla at the Planetary Society has an excellent writeup of how this works. This technique has been used on many asteroids to determine their size, shape, and rotation (see Related Posts below). It’s best if the object passes relatively close to Earth, and Arecibo took these observations when LINEAR was only a few million kilometers away.

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!

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In the images reconstructed from the radar observations, what’s shown as the bottom of the comet nucleus is the part facing Earth (so it’s like the Earth is off the bottom of the frame, and we’re looking up at it). There are a lot of contours visible, possibly cliffs or large ridges. There’s not a lot of detail in these first pictures, but further analysis over the coming weeks should reveal more.

The biggest surprise to me was the size of the comet: Arecibo found it to be elongated, about 2.4 x 3 miles (3.9 x 4.8 kilometers) across.* The elongated shape is somewhat expected; most comet nuclei are oddly shaped. But it’s far bigger than I expected; in a paper I read based on other observations the size was estimated to be about 600 meters (2,000 feet). The radar observations are pretty definitive, so that means we still have some things to figure out in converting brightnesses of comets to their physical size.

But then that’s really hard to do; comets are chunks of ice and rock, and the ice turns to gas as they approach the Sun. The envelope of gas, called the coma, can be very thick and bright, obscuring the solid nucleus. Not only that, the coma can be thousands of kilometers across (bigger than planets), while the nucleus may be the size of a large mountain. Even at their closest, comet nuclei would be dots in our telescopes, so that together with the obscuration by the coma makes determining their true size extremely difficult.

Over the years, Arecibo has looked at only five comets (including this one), and these are the highest resolution images it’s yet obtained (due to the proximity of the comet, which passed about 9 million kilometers from Earth in May). We’ve also visited eight comets with spacecraft (soon to be nine!), so images like these are critical in our understanding of how these beasties tick.

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